kerravonsen | Acrylic Pouring and Paint Density

Someone in one of the acrylic pouring groups I'm in asked "So how do you determine which paint is heaviest?" I started answering in a comment, but it got longer and longer, and I figured it would be helpful to other people as well, so I'm doing this as a separate post.

First, some context, for those of you who don't know what the heck I'm talking about.

Acrylic pouring is a type of fluid art, where the artist takes acrylic paints and thins them down (with a medium) until they are thin enough to pour onto their canvas(+), rather than being applied with a brush. Other types of fluid art use different materials, such as resin, but the principle is the same: to pour your colours onto a surface, and manipulate them in various ways, whether that be tilting, blowing, swiping, combing, squishing... whatever the imagination can come up with to move the fluid around to create a composition, as the artist desires. The result is something which has a very organic feel, because the movement of the paint is more "natural" than applying it with a brush.

Now, one of the things which is desirable in fluid art is the formation of "cells", where two or more colours are layered on top of each other, and the colour(s) underneath well up to the surface, creating a "cell" with different colour(s) than the surface colour. Two things are needed for the formation of cells: movement, and a difference in density. The "movement" is provided by the artist, whether that be in the way the paint is initially added to the surface (e.g. a flip cup) or manipulated afterwards (e.g. a swipe). The difference in density is needed because a less dense (lighter) colour needs to rise and/or a more dense (heavier) colour needs to sink in order create the cells. There are two main ways of creating a difference in density:

putting additives in the paint to create small areas of lighter density, or
paying attention to the density of the paint itself.

With the first method, just before a colour is added to the canvas, a small amount of additive (silicone oil / alcohol / detergent / air) is added to one or more colours and stirred in, so that there are small pockets of lighter density in that paint. Then after the paint is on the canvas, a butane torch can be used to lightly heat the surface to encourage the spots of lighter density to come to the surface, and with them bring the paint that they were mixed with.

With the second method, the colours have to be layered so that, when they arrive on the canvas, the most dense colours are on top, and the least dense colours are on the bottom, so that (because the paint is fluid, and thus can move freely) the heavier colours will sink, and the lighter colours will float, thus creating cells. The greater the difference in density, the more likely it is that cells will form. Thus, with this method, where the density is just that of the paint, rather than, say, oil (which always floats on water), it isn't as easy to create cells as it is with additives, because the difference in density isn't as large. However, this method has the advantage of not contaminating the paint with additives that might threaten the integrity of the paint (such as detergent) or require a painstaking clean-up after the painting is dry (such as oil).

So, how do you determine which paint is the heaviest (densest)?

Within the same brand of paint, the density is determined by the pigments used to colour the paint, since that is the only thing different between each colour. Across different brands, the paint is made of different materials, so you can't be certain that the density will the same as with a different brand, but the pigment is still the dominant factor, so I think it makes a good rule of thumb for figuring out the relative heaviness of the different paint colours.

With artist-grade paints, they tell you what pigments are used in each colour, with a pigment code. For example, Titanium White is PW6, and Carbon Black is PBk7. Then you can look up information on that pigment. Note that you cannot always go by the colour name of the paint, because (with some exceptions) different paint brands may use exactly the same name for a colour, and put different pigments into it. The chief exeptions are Titanium White (PW6), Carbon Black (PBk7) and Mars Black (PBk11) -- everyone who uses those names for a paint will be using those precise pigments. Some colours have common names, but the common names aren't always used. For example Quinacridone Magenta uses pigment PR122, but the Jo Sonja brand call the colour using that same pigment "Transparent Magenta". The really confusing bit is with some of the less common colours. For example, the Derivan Matisse colour "Cobalt Turquoise" uses pigment PB36, while the Vallejo colour of the same name uses PB35 plus PY35 (a mix of a blue and a yellow). The other thing to keep in mind is that, for some pigments, because of the way the manufacturer makes the paint, using the same pigment doesn't necessarily mean the paint is the same colour, especially within the same brand. For example, the pigment PBr7 is the sole pigment used in Jo Sonja's Burnt Umber, Brown Earth, Burnt Sienna, Raw Sienna, and Raw Umber; all by the same manufacturer, but all distinctly different colours. Another example is the pigment PV19, which is the sole pigment used in Jo Sonja's "Red Violet" and Sennelier's "Primary Red", but they are clearly different colours; the Red Violet is more violet, while the Primary Red is more red. On the other hand, all the Quinacridone Magenta (PR122) paints I've used have been identical in colour. You just have to read the labels carefully.

So, where do you find information about the density of the pigments? Fortunately, Golden Acrylics actually have the density information for their paints freely available (++). What I've done is use the Golden Acrylics information, knowing that it isn't exact when applied to a different brand, but using it as a ballpark figure. It's okay, because you don't have to know the exact density, you just have to be able to figure out what order to put your paints in, relative to each other. For pigments which aren't in the Golden Acrylics document, I've scoured the internet for density information, again knowing that it's a ballpark figure.

I try to stick to colours which have only one pigment in them, rather than a mix of pigments, because the greater number of different pigments there are making up a colour, the more the "ballpark figure" turns into a "wild guess" as far as the density is concerned. The one exception would be a colour which is mixed with pigments of identical density, such as some versions of "Unbleached Titanium"/"Titan Buff" which are a mix of Titanium White (PW6) and Yellow Oxide (PY42) which both have the same density.

For Aussies, there are five brands of acrylic paint I've found (and used) which are relatively inexpensive which do give you the pigment information: Global High Flow Acrylics, Jo Sonja Acrylics, Liquitex Basics Acrylics, Derivan Matisse Flow Acrylics, and Sennelier Abstract Acrylics. The Global is only available from Riot Art & Craft stores (including their website). The Jo Sonja is available from both Riot and Lincraft, and probably other places too. The Liquitex Basics I bought from Lincraft, but I expect they are elsewhere as well. The Derivan Matisse Flow and the Sennelier Abstract is available through the Officeworks website (though not in-store). You can get them from art specialty stores too, of course, but Officeworks is cheaper. If you want to Buy Australian, go for the Global and the Derivan, they are both Australian brands. The Derivan Matisse Flow is the most expensive of the five, but it also has the greatest range of single-pigment colours.

If you don't have pigment information about your paint (because the manufacturer didn't give that information), do not fear, all is not lost! You can use this rule of thumb: White is heavy. This works because the most commonly used white pigment (Titanium White) is quite heavy, and the less commonly used white (Zinc White) is even heavier. Just be aware that Mars Black is a little heavier than Titanium White, while Carbon Black is a lot lighter, so if you're dealing with unknown paints, it may be wiser not to use both black and white in the same painting. (Or use just black and white and have fun experimenting!)

(+) When I say "canvas" I mean a generic surface to which paint can be applied. It doesn't have to be a traditional artists' canvas, it can be anything which is sufficiently waterproof that it won't buckle or warp with all the liquid paint which is being poured on it. People have successfully poured onto ceramic tiles, acetate sheets, plastic-coated paper, terracotta pots, glass, sealed wood etc.

(++) Another document I've just recently found which contains useful pigment density information is the Sun Chemical Color and Polymer Suitability Guide

ETA: follow-up article with tables of figures